The technology of fluid pressure cores used by the industry in curing new tires and retreading old tires is described generally in C.V. Toddy, U.S. Pat. No. 2,937,406. In this technique, fluid pressure cores, such as full circle or sectional air bags, bladders, and the like, are inserted within uncured or green tires during vulcanization to force the external surface of the article into engagement with the mold by exerting pressure on the internal surface of the uncured tire. The Toddy patent discloses that lubrication of the interface between the external surface of the pressure core, or bag, and the internal surface of the tire is critical because of the high frictional forces developed as the bag expands against the inside surface of the tire. Unless relieved, these forces lead to a substantial reduction of thickness in the bag, particularly in the shoulder areas of the tire, and eventually cause premature failure of the bag. Toddy proposed to overcome this friction by molding integral lubricated rubber layers into the air bag. This method, however, requires specialized molding techniques and is difficult and expensive to accomplish.
Recent developments in mold release formulations have centered on utilizing formulas consisting of fluid lubricants, dispersing aids and particulate matter of a multifunctional nature. These have been employed to lubricate the interface between the external surface of the inflatable rubber bags and the internal surface of the green tire. Previously, materials such as talc, mica and clays of various sorts were used either dry or in suspensions to lubricate the inflatable rubber bag and thus minimize friction by providing at least a small separation between the soft yielding "green stock" and the expanding bag. Silicone oils have also been used to aid in the lubrication of the inflatable bag, and it was found that the more viscous the silicone the better it performed as a green tire lubricant.
In an effort to utilize the advantages of all of these substances, formulations were developed which consisted of a mixture of silicones, polyols and talc or mica clays. Such a formulation for a bag release agent is disclosed in Cekada, U.S. Pat. No. 3,532,624, in which a composition comprising a silicone fluid, a polyol, a particulate mica, and 25 to 55% by weight of a hydrocarbon solvent is sprayed on the surface of a conventional bag, i.e., one which does not include lubricated strips, just before expansion into the green tire.
A disadvantage of release compositions such as the Cekada formulation is that they provide poor wet-out, that is, the lubricant composition does not spread out evenly on the surface of the green tire. An ideal release agent should provide good wet-out, prolong bladder life, release effectively, both initially and after long term aging and cut down on defects, such as air bleed problems, and the like.
There has now been discovered a family of compositions based on silicone fluids which provide the desired advantages, i.e., good wet-out, prolonged bladder life, effective release, and reduction of air bleed problems.
One key feature is to add a small effective amount of an unsaturated fatty acid, e.g., oleic acid, which quite surprisingly improves both the wet-out and the lubricity of the composition.
A second key feature is to use a ratio of combined silicone and polyol to particulate solid which is greater than that ordinarily thought necessary, e.g., at least about 0.6:1, on a weight basis. Surprisingly, apparently because of the presence of unsaturated fatty acid in the composition, the materials still retain high air bleed efficiency at the somewhat lower particulate solids content.
In addition to outstanding lubricity, the new compositions have very good wet-out.
Accordingly, it is a primary object of this invention to provide a high lubricity composition with very good wet-out, using greater than the usual ratios of siloxanes and polyols to particulate solid material.
It is also an object to provide such compositions which include an unsaturated fatty acid additive to improve wet-out and lubricity, while maintaining air-bleed capability.